Salmon Killer Disease Mystery Solved

The identity of a mysterious disease that’s raged through European salmon farms, wasting the hearts and muscles of infected fish, has been revealed.

Genome sleuthing shows the disease is caused by a previously unknown virus. The identification doesn’t suggest an obvious cure — for now, scientists have only a name and a genome — but it’s an important first step.

“It’s a new virus. And with this information now in hand, we can make vaccines,” said Ian Lipkin, director of Columbia University’s Center for Infection and Immunity, a World Health Organization-sponsored disease detective lab.

Two years ago, Norweigan fisheries scientists approached Lipkin and asked for help in identifying the cause of Heart and Skeletal Muscle Inflammation, or HSMI, the official name for a disease first identified in 1999 on a Norweigan salmon farm.

Infected fish are physically stunted, and their muscles are so weakened that they have trouble swimming or even pumping blood. The disease is often fatal, and the original outbreak has been followed by 417 others in Norway and the United Kingdom. Every year there’s more of the disease, and it’s now been seen in wild fish, suggesting that farm escapees are infecting already-dwindling wild stocks.

Lipkin’s team — which has also identified mystery viruses killing Great Apes in the Ivory Coast, and sea lions off the U.S. West Coast — combed through genetic material sampled from infection salmon pens, looking for DNA sequences resembling what’s seen in other viruses, and inferring from those what the HSMI-causing sequence should look like. Lipkin likened the process to solving a crossword puzzle. The researchers eventually arrived at the 10-gene virus they called piscine reovirus, or PRV. The virus was described July 9 in Public Library of Science One.

Related reoviruses have been found on poultry farms and cause muscle and heart disease in chickens. “Analogies between commercial poultry production and Atlantic salmon aquaculture may be informative,” wrote the researchers. “Both poultry production and aquaculture confine animals at high density in conditions that are conducive to transmission of infectious agents.”